Method and apparatus for selecting a sensing vector configuration in a medical device

ABSTRACT

A method and medical device for determining sensing vectors that includes sensing cardiac signals from a plurality of electrodes, the plurality of electrodes forming a plurality of sensing vectors, determining a sensing vector metric in response to the sensed cardiac signals, determining a morphology metric associated with a morphology of the sensed cardiac signals, determining vector selection metrics in response to the determined sensing vector metric and the determined morphology setting, and selecting a sensing vector of the plurality of sensing vectors in response to the determined vector selection metrics.

RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.14/339,980, filed Jul. 24, 2014, which claims priority and otherbenefits from U.S. Provisional Patent Application Ser. No. 61/983,499,filed Apr. 24, 2014, entitled “METHOD AND APPARATUS FOR SELECTING ASENSING VECTOR CONFIGURATION IN A MEDICAL DEVICE”, both of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to an apparatus and method for selecting a sensing vector ina medical device.

BACKGROUND

Implantable medical devices are available for preventing or treatingcardiac arrhythmias by delivering anti-tachycardia pacing therapies andelectrical shock therapies for cardioverting or defibrillating theheart. Such a device, commonly known as an implantable cardioverterdefibrillator or “ICD”, senses a patient's heart rhythm and classifiesthe rhythm according to a number of rate zones in order to detectepisodes of tachycardia or fibrillation.

Upon detecting an abnormal rhythm, the ICD delivers an appropriatetherapy. Pathologic forms of ventricular tachycardia can often beterminated by anti-tachycardia pacing therapies. Anti-tachycardia pacingtherapies are followed by high-energy shock therapy when necessary.Termination of a tachycardia by a shock therapy is commonly referred toas “cardioversion.” Ventricular fibrillation (VF) is a form oftachycardia that is a serious life-threatening condition and is normallytreated by immediately delivering high-energy shock therapy. Terminationof VF is commonly referred to as “defibrillation.” Accurate arrhythmiadetection and discrimination are important in selecting the appropriatetherapy for effectively treating an arrhythmia and avoiding the deliveryof unnecessary cardioversion/defibrillation (CV/DF) shocks, which arepainful to the patient.

In past practice, ICD systems have employed intra-cardiac electrodescarried by transvenous leads for sensing cardiac electrical signals anddelivering electrical therapies. Emerging ICD systems are adapted forsubcutaneous or submuscular implantation and employ electrodesincorporated on the ICD housing and/or carried by subcutaneous orsubmuscular leads. These systems, referred to generally herein as“subcutaneous ICD” or “SubQ ICD” systems, do not rely on electrodesimplanted in direct contact with the heart. SubQ ICD systems are lessinvasive and are therefore implanted more easily and quickly than ICDsystems that employ intra-cardiac electrodes. However, greaterchallenges exist in reliably detecting cardiac arrhythmias using asubcutaneous system. The R-wave amplitude on a SubQ ECG signal may be onthe order of one-tenth to one-one hundredth of the amplitude ofintra-ventricular sensed R-waves. Furthermore, the signal quality ofsubcutaneously sensed ECG signals are likely to be more affected bymyopotential noise, environmental noise, patient posture and patientactivity than intra-cardiac myocardial electrogram (EGM) signals.

The ability of a subcutaneous ICD to detect tachyarrhythmias and rejectnoise depends on its ECG signal characteristics. ECG vectors with higheramplitude R-wave waves, higher frequency (high slew rate) R-waves,higher R/T wave ratios, lower frequency signal (e.g., P and T waves)around R-waves, lower susceptibility to skeletal myopotentials, andgreater R-wave consistency from cycle to cycle are preferred to ECGvectors without these attributes. A subcutaneous ICD with a minimum of 2ECG leads or vectors (using a minimum of 3 electrodes) in a plane mayuse these physical vectors to generate virtual ECG vectors using alinear combination of the physical vector ECGs. However, choosing theoptimal vector may sometimes be a challenge given the changingenvironment of a subcutaneous system. As such, systems and methods thatpromote reliable and accurate sensing detection of arrhythmias usingoptimal available sensing vectors to sense ECG signals via subcutaneouselectrodes are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a patient implanted with an exampleextravascular cardiac defibrillation system.

FIG. 2 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention.

FIG. 3 is a flowchart of a method for selecting a sensing vector in amedical device, according to one embodiment.

FIG. 4 is a graphical representation of cardiac signals sensed alongmultiple sensing vectors during selection of a sensing vector in amedical device according to one embodiment.

FIG. 5 is a flowchart of a method for determining a morphology metricfor selecting a sensing vector, according to one embodiment.

FIG. 6 is a chart illustrating a method of utilizing determinedselection metrics for selecting a sensing vector, according to anexemplary embodiment.

FIG. 7 is a flowchart of a method for selecting sensing vectors usingdetermined vector selection metrics and morphology selection metricsaccording to an embodiment.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram of a patient 12 implanted with an exampleextravascular cardiac defibrillation system 10. In the exampleillustrated in FIG. 1, extravascular cardiac defibrillation system 10 isan implanted subcutaneous ICD system. However, the techniques of thisdisclosure may also be utilized with other extravascular implantedcardiac defibrillation systems, such as a cardiac defibrillation systemhaving a lead implanted at least partially in a substernal orsubmuscular location. Additionally, the techniques of this disclosuremay also be utilized with other implantable systems, such as implantablepacing systems, implantable neurostimulation systems, drug deliverysystems or other systems in which leads, catheters or other componentsare implanted at extravascular locations within patient 12. Thisdisclosure, however, is described in the context of an implantableextravascular cardiac defibrillation system for purposes ofillustration.

Extravascular cardiac defibrillation system 10 includes an implantablecardioverter defibrillator (ICD) 14 connected to at least oneimplantable cardiac defibrillation lead 16. ICD 14 of FIG. 1 isimplanted subcutaneously on the left side of patient 12. Defibrillationlead 16, which is connected to ICD 14, extends medially from ICD 14toward sternum 28 and xiphoid process 24 of patient 12. At a locationnear xiphoid process 24, defibrillation lead 16 bends or turns andextends subcutaneously superior, substantially parallel to sternum 28.In the example illustrated in FIG. 1, defibrillation lead 16 isimplanted such that lead 16 is offset laterally to the left side of thebody of sternum 28 (i.e., towards the left side of patient 12).

Defibrillation lead 16 is placed along sternum 28 such that a therapyvector between defibrillation electrode 18 and a second electrode (suchas a housing or can 25 of ICD 14 or an electrode placed on a secondlead) is substantially across the ventricle of heart 26. The therapyvector may, in one example, be viewed as a line that extends from apoint on the defibrillation electrode 18 to a point on the housing orcan 25 of ICD 14. In another example, defibrillation lead 16 may beplaced along sternum 28 such that a therapy vector betweendefibrillation electrode 18 and the housing or can 25 of ICD 14 (orother electrode) is substantially across an atrium of heart 26. In thiscase, extravascular ICD system 10 may be used to provide atrialtherapies, such as therapies to treat atrial fibrillation.

The embodiment illustrated in FIG. 1 is an example configuration of anextravascular ICD system 10 and should not be considered limiting of thetechniques described herein. For example, although illustrated as beingoffset laterally from the midline of sternum 28 in the example of FIG.1, defibrillation lead 16 may be implanted such that lead 16 is offsetto the right of sternum 28 or more centrally located over sternum 28.Additionally, defibrillation lead 16 may be implanted such that it isnot substantially parallel to sternum 28, but instead offset fromsternum 28 at an angle (e.g., angled lateral from sternum 28 at eitherthe proximal or distal end). As another example, the distal end ofdefibrillation lead 16 may be positioned near the second or third rib ofpatient 12. However, the distal end of defibrillation lead 16 may bepositioned further superior or inferior depending on the location of ICD14, location of electrodes 18, 20, and 22, or other factors.

Although ICD 14 is illustrated as being implanted near a midaxillaryline of patient 12, ICD 14 may also be implanted at other subcutaneouslocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which ICD 14 is implanted pectorally, lead16 would follow a different path, e.g., across the upper chest area andinferior along sternum 28. When the ICD 14 is implanted in the pectoralregion, the extravascular ICD system may include a second lead includinga defibrillation electrode that extends along the left side of thepatient such that the defibrillation electrode of the second lead islocated along the left side of the patient to function as an anode orcathode of the therapy vector of such an ICD system.

ICD 14 includes a housing or can 25 that forms a hermetic seal thatprotects components within ICD 14. The housing 25 of ICD 14 may beformed of a conductive material, such as titanium or other biocompatibleconductive material or a combination of conductive and non-conductivematerials. In some instances, the housing 25 of ICD 14 functions as anelectrode (referred to as a housing electrode or can electrode) that isused in combination with one of electrodes 18, 20, or 22 to deliver atherapy to heart 26 or to sense electrical activity of heart 26. ICD 14may also include a connector assembly (sometimes referred to as aconnector block or header) that includes electrical feedthroughs throughwhich electrical connections are made between conductors withindefibrillation lead 16 and electronic components included within thehousing. Housing may enclose one or more components, includingprocessors, memories, transmitters, receivers, sensors, sensingcircuitry, therapy circuitry and other appropriate components (oftenreferred to herein as modules).

Defibrillation lead 16 includes a lead body having a proximal end thatincludes a connector configured to connect to ICD 14 and a distal endthat includes one or more electrodes 18, 20, and 22. The lead body ofdefibrillation lead 16 may be formed from a non-conductive material,including silicone, polyurethane, fluoropolymers, mixtures thereof, andother appropriate materials, and shaped to form one or more lumenswithin which the one or more conductors extend. However, the techniquesare not limited to such constructions. Although defibrillation lead 16is illustrated as including three electrodes 18, 20 and 22,defibrillation lead 16 may include more or fewer electrodes.

Defibrillation lead 16 includes one or more elongated electricalconductors (not illustrated) that extend within the lead body from theconnector on the proximal end of defibrillation lead 16 to electrodes18, 20 and 22. In other words, each of the one or more elongatedelectrical conductors contained within the lead body of defibrillationlead 16 may engage with respective ones of electrodes 18, 20 and 22.When the connector at the proximal end of defibrillation lead 16 isconnected to ICD 14, the respective conductors may electrically coupleto circuitry, such as a therapy module or a sensing module, of ICD 14via connections in connector assembly, including associatedfeedthroughs. The electrical conductors transmit therapy from a therapymodule within ICD 14 to one or more of electrodes 18, 20 and 22 andtransmit sensed electrical signals from one or more of electrodes 18, 20and 22 to the sensing module within ICD 14.

ICD 14 may sense electrical activity of heart 26 via one or more sensingvectors that include combinations of electrodes 20 and 22 and thehousing or can 25 of ICD 14. For example, ICD 14 may obtain electricalsignals sensed using a sensing vector between electrodes 20 and 22,obtain electrical signals sensed using a sensing vector betweenelectrode 20 and the conductive housing or can 25 of ICD 14, obtainelectrical signals sensed using a sensing vector between electrode 22and the conductive housing or can 25 of ICD 14, or a combinationthereof. In some instances, ICD 14 may sense cardiac electrical signalsusing a sensing vector that includes defibrillation electrode 18, suchas a sensing vector between defibrillation electrode 18 and one ofelectrodes 20 or 22, or a sensing vector between defibrillationelectrode 18 and the housing or can 25 of ICD 14.

ICD may analyze the sensed electrical signals to detect tachycardia,such as ventricular tachycardia or ventricular fibrillation, and inresponse to detecting tachycardia may generate and deliver an electricaltherapy to heart 26. For example, ICD 14 may deliver one or moredefibrillation shocks via a therapy vector that includes defibrillationelectrode 18 of defibrillation lead 16 and the housing or can 25.Defibrillation electrode 18 may, for example, be an elongated coilelectrode or other type of electrode. In some instances, ICD 14 maydeliver one or more pacing therapies prior to or after delivery of thedefibrillation shock, such as anti-tachycardia pacing (ATP) or postshock pacing. In these instances, ICD 14 may generate and deliver pacingpulses via therapy vectors that include one or both of electrodes 20 and22 and/or the housing or can 25. Electrodes 20 and 22 may comprise ringelectrodes, hemispherical electrodes, coil electrodes, helix electrodes,segmented electrodes, directional electrodes, or other types ofelectrodes, or combination thereof. Electrodes 20 and 22 may be the sametype of electrodes or different types of electrodes, although in theexample of FIG. 1 both electrodes 20 and 22 are illustrated as ringelectrodes.

Defibrillation lead 16 may also include an attachment feature 29 at ortoward the distal end of lead 16. The attachment feature 29 may be aloop, link, or other attachment feature. For example, attachment feature29 may be a loop formed by a suture. As another example, attachmentfeature 29 may be a loop, link, ring of metal, coated metal or apolymer. The attachment feature 29 may be formed into any of a number ofshapes with uniform or varying thickness and varying dimensions.Attachment feature 29 may be integral to the lead or may be added by theuser prior to implantation. Attachment feature 29 may be useful to aidin implantation of lead 16 and/or for securing lead 16 to a desiredimplant location. In some instances, defibrillation lead 16 may includea fixation mechanism in addition to or instead of the attachmentfeature. Although defibrillation lead 16 is illustrated with anattachment feature 29, in other examples lead 16 may not include anattachment feature 29.

Lead 16 may also include a connector at the proximal end of lead 16,such as a DF4 connector, bifurcated connector (e.g., DF-1/IS-1connector), or other type of connector. The connector at the proximalend of lead 16 may include a terminal pin that couples to a port withinthe connector assembly of ICD 14. In some instances, lead 16 may includean attachment feature at the proximal end of lead 16 that may be coupledto an implant tool to aid in implantation of lead 16. The attachmentfeature at the proximal end of the lead may separate from the connectorand may be either integral to the lead or added by the user prior toimplantation.

Defibrillation lead 16 may also include a suture sleeve or otherfixation mechanism (not shown) located proximal to electrode 22 that isconfigured to fixate lead 16 near the xiphoid process or lower sternumlocation. The fixation mechanism (e.g., suture sleeve or othermechanism) may be integral to the lead or may be added by the user priorto implantation.

The example illustrated in FIG. 1 is exemplary in nature and should notbe considered limiting of the techniques described in this disclosure.For instance, extravascular cardiac defibrillation system 10 may includemore than one lead. In one example, extravascular cardiac defibrillationsystem 10 may include a pacing lead in addition to defibrillation lead16.

In the example illustrated in FIG. 1, defibrillation lead 16 isimplanted subcutaneously, e.g., between the skin and the ribs orsternum. In other instances, defibrillation lead 16 (and/or the optionalpacing lead) may be implanted at other extravascular locations. In oneexample, defibrillation lead 16 may be implanted at least partially in asubsternal location. In such a configuration, at least a portion ofdefibrillation lead 16 may be placed under or below the sternum in themediastinum and, more particularly, in the anterior mediastinum. Theanterior mediastinum is bounded laterally by pleurae, posteriorly bypericardium, and anteriorly by sternum 28. Defibrillation lead 16 may beat least partially implanted in other extra-pericardial locations, i.e.,locations in the region around, but not in direct contact with, theouter surface of heart 26. These other extra-pericardial locations mayinclude in the mediastinum but offset from sternum 28, in the superiormediastinum, in the middle mediastinum, in the posterior mediastinum, inthe sub-xiphoid or inferior xiphoid area, near the apex of the heart, orother location not in direct contact with heart 26 and not subcutaneous.In still further instances, the lead may be implanted at a pericardialor epicardial location outside of the heart 26.

FIG. 2 is an exemplary schematic diagram of electronic circuitry withina hermetically sealed housing of a subcutaneous device according to anembodiment of the present invention. As illustrated in FIG. 2,subcutaneous device 14 includes a low voltage battery 153 coupled to apower supply (not shown) that supplies power to the circuitry of thesubcutaneous device 14 and the pacing output capacitors to supply pacingenergy in a manner well known in the art. The low voltage battery 153may be formed of one or two conventional LiCF_(x), LiMnO₂ or LiI₂ cells,for example. The subcutaneous device 14 also includes a high voltagebattery 112 that may be formed of one or two conventional LiSVO orLiMnO₂ cells. Although two both low voltage battery and a high voltagebattery are shown in FIG. 2, according to an embodiment of the presentinvention, the device 14 could utilize a single battery for both highand low voltage uses.

Further referring to FIG. 2, subcutaneous device 14 functions arecontrolled by means of software, firmware and hardware thatcooperatively monitor the ECG signal, determine when acardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. Thesubcutaneous device 14 may incorporate circuitry set forth in commonlyassigned U.S. Pat. No. 5,163,427 “Apparatus for Delivering Single andMultiple Cardioversion and Defibrillation Pulses” to Keimel and U.S.Pat. No. 5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia”to Keimel for selectively delivering single phase, simultaneous biphasicand sequential biphasic cardioversion-defibrillation shocks typicallyemploying ICD IPG housing electrodes 28 coupled to the COMMON output 123of high voltage output circuit 140 and cardioversion-defibrillationelectrode 24 disposed posterially and subcutaneously and coupled to theHVI output 113 of the high voltage output circuit 140.

The cardioversion-defibrillation shock energy and capacitor chargevoltages can be intermediate to those supplied by ICDs having at leastone cardioversion-defibrillation electrode in contact with the heart andmost AEDs having cardioversion-defibrillation electrodes in contact withthe skin. The typical maximum voltage necessary for ICDs using mostbiphasic waveforms is approximately 750 Volts with an associated maximumenergy of approximately 40 Joules. The typical maximum voltage necessaryfor AEDs is approximately 2000-5000 Volts with an associated maximumenergy of approximately 200-360 Joules depending upon the model andwaveform used. The subcutaneous device 14 of the present invention usesmaximum voltages in the range of about 300 to approximately 1500 Voltsand is associated with energies of approximately 25 to 150 joules ormore. The total high voltage capacitance could range from about 50 toabout 300 microfarads. Such cardioversion-defibrillation shocks are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying the detection algorithms as described herein below.

In FIG. 2, sense amp 190 in conjunction with pacer/device timing circuit178 processes the far field ECG sense signal that is developed across aparticular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 18, 20, 22 and the can or housing 25 of thedevice 14, or, optionally, a virtual signal (i.e., a mathematicalcombination of two vectors) if selected. For example, the device maygenerate a virtual vector signal as described in U.S. Pat. No. 6,505,067“System and Method for Deriving Virtual ECG or EGM Signal” to Lee, etal; both patents incorporated herein by reference in their entireties.In addition, vector selection may be selected by the patient's physicianand programmed via a telemetry link from a programmer.

The selection of the sensing electrode pair is made through the switchmatrix/MUX 191 in a manner to provide the most reliable sensing of theECG signal of interest, which would be the R wave for patients who arebelieved to be at risk of ventricular fibrillation leading to suddendeath. The far field ECG signals are passed through the switchmatrix/MUX 191 to the input of the sense amplifier 190 that, inconjunction with pacer/device timing circuit 178, evaluates the sensedEGM. Bradycardia, or asystole, is typically determined by an escapeinterval timer within the pacer timing circuit 178 and/or the controlcircuit 144. Pace Trigger signals are applied to the pacing pulsegenerator 192 generating pacing stimulation when the interval betweensuccessive R-waves exceeds the escape interval. Bradycardia pacing isoften temporarily provided to maintain cardiac output after delivery ofa cardioversion-defibrillation shock that may cause the heart to slowlybeat as it recovers back to normal function. Sensing subcutaneous farfield signals in the presence of noise may be aided by the use ofappropriate denial and extensible accommodation periods as described inU.S. Pat. No. 6,236,882 “Noise Rejection for Monitoring ECGs” to Lee, etal and incorporated herein by reference in its' entirety.

Detection of a malignant tachyarrhythmia is determined in the Controlcircuit 144 as a function of the intervals between R-wave sense eventsignals that are output from the pacer/device timing 178 and senseamplifier circuit 190 to the timing and control circuit 144. It shouldbe noted that the present invention utilizes not only interval basedsignal analysis method but also supplemental sensors and morphologyprocessing method and apparatus as described herein below.

Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt and incorporated hereinby reference in its entirety. Sensor processing block 194 providessensor data to microprocessor 142 via data bus 146. Specifically,patient activity and/or posture may be determined by the apparatus andmethod as described in U.S. Pat. No. 5,593,431 “Medical ServiceEmploying Multiple DC Accelerometers for Patient Activity and PostureSensing and Method” to Sheldon and incorporated herein by reference inits entirety. Patient respiration may be determined by the apparatus andmethod as described in U.S. Pat. No. 4,567,892 “Implantable CardiacPacemaker” to Plicchi, et al and incorporated herein by reference in itsentirety. Patient tissue oxygenation or tissue color may be determinedby the sensor apparatus and method as described in U.S. Pat. No.5,176,137 to Erickson, et al and incorporated herein by reference in itsentirety. The oxygen sensor of the '137 patent may be located in thesubcutaneous device pocket or, alternatively, located on the lead 18 toenable the sensing of contacting or near-contacting tissue oxygenationor color.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 142, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown)conventional in the art. Data and commands are exchanged betweenmicrocomputer 142 and timing and control circuit 144, pacertiming/amplifier circuit 178, and high voltage output circuit 140 via abi-directional data/control bus 146. The pacer timing/amplifier circuit178 and the control circuit 144 are clocked at a slow clock rate. Themicrocomputer 142 is normally asleep, but is awakened and operated by afast clock by interrupts developed by each R-wave sense event, onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pacer/device timing circuitry 178.

When a malignant tachycardia is detected, high voltage capacitors 156,158, 160, and 162 are charged to a pre-programmed voltage level by ahigh-voltage charging circuit 164. It is generally consideredinefficient to maintain a constant charge on the high voltage outputcapacitors 156, 158, 160, 162. Instead, charging is initiated whencontrol circuit 144 issues a high voltage charge command HVCHG deliveredon line 145 to high voltage charge circuit 164 and charging iscontrolled by means of bi-directional control/data bus 166 and afeedback signal VCAP from the HV output circuit 140. High voltage outputcapacitors 156, 158, 160 and 162 may be of film, aluminum electrolyticor wet tantalum construction.

The negative terminal of high voltage battery 112 is directly coupled tosystem ground. Switch circuit 114 is normally open so that the positiveterminal of high voltage battery 112 is disconnected from the positivepower input of the high voltage charge circuit 164. The high voltagecharge command HVCHG is also conducted via conductor 149 to the controlinput of switch circuit 114, and switch circuit 114 closes in responseto connect positive high voltage battery voltage EXT B+ to the positivepower input of high voltage charge circuit 164. Switch circuit 114 maybe, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+ conductor 118 and its gatereceiving the HVCHG signal on conductor 145. High voltage charge circuit164 is thereby rendered ready to begin charging the high voltage outputcapacitors 156, 158, 160, and 162 with charging current from highvoltage battery 112.

High voltage output capacitors 156, 158, 160, and 162 may be charged tovery high voltages, e.g., 300-1500V, to be discharged through the bodyand heart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 113 and 123. The details of thevoltage charging circuitry are also not deemed to be critical withregard to practicing the present invention; one high voltage chargingcircuit believed to be suitable for the purposes of the presentinvention is disclosed. High voltage capacitors 156, 158, 160 and 162may be charged, for example, by high voltage charge circuit 164 and ahigh frequency, high-voltage transformer 168 as described in detail incommonly assigned U.S. Pat. No. 4,548,209 “Energy Converter forImplantable Cardioverter” to Wielders, et al. Proper charging polaritiesare maintained by diodes 170, 172, 174 and 176 interconnecting theoutput windings of high-voltage transformer 168 and the capacitors 156,158, 160, and 162. As noted above, the state of capacitor charge ismonitored by circuitry within the high voltage output circuit 140 thatprovides a VCAP, feedback signal indicative of the voltage to the timingand control circuit 144. Timing and control circuit 144 terminates thehigh voltage charge command HVCHG when the VCAP signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 144 then develops first and second control signalsNPULSE 1 and NPULSE 2, respectively, that are applied to the highvoltage output circuit 140 for triggering the delivery of cardiovertingor defibrillating shocks. In particular, the NPULSE 1 signal triggersdischarge of the first capacitor bank, comprising capacitors 156 and158. The NPULSE 2 signal triggers discharge of the first capacitor bankand a second capacitor bank, comprising capacitors 160 and 162. It ispossible to select between a plurality of output pulse regimes simply bymodifying the number and time order of assertion of the NPULSE 1 andNPULSE 2 signals. The NPULSE 1 signals and NPULSE 2 signals may beprovided sequentially, simultaneously or individually. In this way,control circuitry 144 serves to control operation of the high voltageoutput stage 140, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 18 and 25 coupled to the HV-1and COMMON output as shown in FIG. 2.

Thus, subcutaneous device 14 monitors the patient's cardiac status andinitiates the delivery of a cardioversion-defibrillation shock throughthe cardioversion-defibrillation electrodes 18 and 25 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 112 to beconnected through the switch circuit 114 with the high voltage chargecircuit 164 and the charging of output capacitors 156, 158, 160, and 162to commence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit144 sets the HVCHG signal low terminating charging and opening switchcircuit 114. The subcutaneous device 14 can be programmed to attempt todeliver cardioversion shocks to the heart in the manners described abovein timed synchrony with a detected R-wave or can be programmed orfabricated to deliver defibrillation shocks to the heart in the mannersdescribed above without attempting to synchronize the delivery to adetected R-wave. Episode data related to the detection of thetachyarrhythmia and delivery of the cardioversion-defibrillation shockcan be stored in RAM for uplink telemetry transmission to an externalprogrammer as is well known in the art to facilitate in diagnosis of thepatient's cardiac state. A patient receiving the device 14 on aprophylactic basis would be instructed to report each such episode tothe attending physician for further evaluation of the patient'scondition and assessment for the need for implantation of a moresophisticated ICD.

Subcutaneous device 14 desirably includes telemetry circuit (not shownin FIG. 2), so that it is capable of being programmed by means ofexternal programmer 20 via a 2-way telemetry link (not shown). Uplinktelemetry allows device status and diagnostic/event data to be sent toexternal programmer 20 for review by the patient's physician. Downlinktelemetry allows the external programmer via physician control to allowthe programming of device function and the optimization of the detectionand therapy for a specific patient. Programmers and telemetry systemssuitable for use in the practice of the present invention have been wellknown for many years. Known programmers typically communicate with animplanted device via a bi-directional radio-frequency telemetry link, sothat the programmer can transmit control commands and operationalparameter values to be received by the implanted device, so that theimplanted device can communicate diagnostic and operational data to theprogrammer. Programmers believed to be suitable for the purposes ofpracticing the present invention include the Models 9790 and CareLink®programmers, commercially available from Medtronic, Inc., Minneapolis,Minn.

Various telemetry systems for providing the necessary communicationschannels between an external programming unit and an implanted devicehave been developed and are well known in the art. Telemetry systemsbelieved to be suitable for the purposes of practicing the presentinvention are disclosed, for example, in the following U.S. Patents:U.S. Pat. No. 5,127,404 to Wyborny et al. entitled “Telemetry Format forImplanted Medical Device”; U.S. Pat. No. 4,374,382 to Markowitz entitled“Marker Channel Telemetry System for a Medical Device”; and U.S. Pat.No. 4,556,063 to Thompson et al. entitled “Telemetry System for aMedical Device”. The Wyborny et al. '404, Markowitz '382, and Thompsonet al. '063 patents are commonly assigned to the assignee of the presentinvention, and are each hereby incorporated by reference herein in theirrespective entireties.

According to an embodiment of the present invention, in order toautomatically select the preferred ECG vector set, it is necessary tohave an index of merit upon which to rate the quality of the signal.“Quality” is defined as the signal's ability to provide accurate heartrate estimation and accurate morphological waveform separation betweenthe patient's usual sinus rhythm and the patient's ventriculartachyarrhythmia.

Appropriate indices may include R-wave amplitude, R-wave peak amplitudeto waveform amplitude between R-waves (i.e., signal to noise ratio), lowslope content, relative high versus low frequency power, mean frequencyestimation, probability density function, or some combination of thesemetrics.

Automatic vector selection might be done at implantation or periodically(daily, weekly, monthly) or both. At implant, automatic vector selectionmay be initiated as part of an automatic device turn-on procedure thatperforms such activities as measure lead impedances and batteryvoltages. The device turn-on procedure may be initiated by theimplanting physician (e.g., by pressing a programmer button) or,alternatively, may be initiated automatically upon automatic detectionof device/lead implantation. The turn-on procedure may also use theautomatic vector selection criteria to determine if ECG vector qualityis adequate for the current patient and for the device and leadposition, prior to suturing the subcutaneous device 14 device in placeand closing the incision. Such an ECG quality indicator would allow theimplanting physician to maneuver the device to a new location ororientation to improve the quality of the ECG signals as required. Thepreferred ECG vector or vectors may also be selected at implant as partof the device turn-on procedure. The preferred vectors might be thosevectors with the indices that maximize rate estimation and detectionaccuracy. There may also be an a priori set of vectors that arepreferred by the physician, and as long as those vectors exceed someminimum threshold, or are only slightly worse than some other moredesirable vectors, the a priori preferred vectors are chosen. Certainvectors may be considered nearly identical such that they are not testedunless the a priori selected vector index falls below some predeterminedthreshold.

Depending upon metric power consumption and power requirements of thedevice, the ECG signal quality metric may be measured on the range ofvectors (or alternatively, a subset) as often as desired. Data may begathered, for example, on a minute, hourly, daily, weekly or monthlybasis. More frequent measurements (e.g., every minute) may be averagedover time and used to select vectors based upon susceptibility ofvectors to occasional noise, motion noise, or EMI, for example.

Alternatively, the subcutaneous device 14 may have an indicator/sensorof patient activity (piezo-resistive, accelerometer, impedance, or thelike) and delay automatic vector measurement during periods of moderateor high patient activity to periods of minimal to no activity. Onerepresentative scenario may include testing/evaluating ECG vectors oncedaily or weekly while the patient has been determined to be asleep(using an internal clock (e.g., 2:00 am) or, alternatively, infer sleepby determining the patient's position (via a 2- or 3-axis accelerometer)and a lack of activity). In another possible scenario, thetesting/evaluating ECG vectors may be performed once daily or weeklywhile the patient is known to be exercising.

If infrequent automatic, periodic measurements are made, it may also bedesirable to measure noise (e.g., muscle, motion, EMI, etc.) in thesignal and postpone the vector selection measurement until a period oftime when the noise has subsided.

Subcutaneous device 14 may optionally have an indicator of the patient'sposture (via a 2- or 3-axis accelerometer). This sensor may be used toensure that the differences in ECG quality are not simply a result ofchanging posture/position. The sensor may be used to gather data in anumber of postures so that ECG quality may be averaged over thesepostures, or otherwise combined, or, alternatively, selected for apreferred posture.

In one embodiment, vector quality metric calculations may be performedby the clinician using a programmer either at the time of implant,during a subsequent visit in a clinic setting, or remotely via a remotelink with the device and the programmer. According to anotherembodiment, the vector quality metric calculations may be performedautomatically for each available sensing vector by the device apredetermined number of times, such multiple times daily, once per day,weekly or on a monthly basis. In addition, the values could be averagedfor each vector over the course of one week, for example. Averaging mayconsist of a moving average or recursive average depending on timeweighting and memory considerations.

FIG. 3 is a flowchart of a method for selecting a sensing vector in amedical device, according to one embodiment. As illustrated in FIG. 3,according to an embodiment of the disclosure, the device senses acardiac signal for each available sensing vector 102-106, using sensingtechniques known in the art, such as described, for example, in U.S.patent application Ser. No. 14/250,040, incorporated herein by referencein it's entirety. The device obtains a sensed R-wave of the cardiacsignal for each available sensing vector 102-106, Block 124, anddetermines both a vector quality metric, Block 126, for determining thequality of a sensing for the vector, and a morphology quality metric,Block 128, for determining the quality of a morphology analysis,associated with the sensed R-wave for that sensing vector 102-106, asdescribed below. Once both a vector quality metric, Block 126 and amorphology metric, Block 128, associated with the sensed R-wave has beendetermined for each sensing vector 102-106, the device determineswhether the vector quality metric and the morphology metric has beendetermined for a predetermined threshold number of cardiac cycles foreach of the sensing vectors 102-106, Block 130. If the vector qualitymetric and the morphology metric has not been determined for thepredetermined threshold number of cardiac cycles for each sensing vector102-106, No in Block 130, the device gets the next R-wave 124 for eachsensing vector 102-106, and the process is repeated for a next sensedcardiac cycle for each of the sensing vectors 102-106. According to oneembodiment, the vector quality metric and the morphology metric isdetermined for 15 cardiac cycles, for example.

Once the vector metric and the morphology metric have been determinedfor the predetermined threshold number of cardiac cycles for eachsensing vector 102-106, Yes in Block 130, the device determinesselection metrics using the determined vector quality metrics andmorphology metrics, Block 132, and selects one or more vectors, Block134, to be utilized during subsequent sensing and arrhythmia detectionby the device based on the determined selection metrics, as describedbelow. Depending on the amount of time programmed to occur betweenupdating of the sensing vectors 102-106, i.e., an hour, day, week ormonth, for example, the device waits until the next scheduled vectorselection determination, Block 136, at which time the vector selectionprocess is repeated.

FIG. 4 is a graphical representation of cardiac signals sensed alongmultiple sensing vectors during selection of a sensing vector in amedical device according to one embodiment. As illustrated in FIG. 4,during the vector selection process, the device senses a cardiac signal100 for each available sensing vector 102-106, using sensing techniquesknown in the art, such as described, for example, in U.S. patentapplication Ser. No. 14/250,040, incorporated herein by reference init's entirety. For example, as illustrated in FIG. 4, according to oneembodiment, the device senses an ECG signal 100 from each of theavailable sensing vectors, including a horizontal sensing vector 102extending between the housing or can 25 and electrode 22, a diagonalsensing vector 104 extending between the housing or can 25 and electrode20, and a vertical sensing vector 106 extending between electrodes 20and 22. The device determines a sensed R-wave 108 for each sensingvector 102-106 as occurring when the sensed signal exceeds atime-dependent self-adjusting sensing threshold 110.

Once the R-wave 108 is sensed, the device determines a vector qualitymetric and a morphology metric for the sensed R-wave, Blocks 126 and 128of FIG. 3. As illustrated in FIG. 4, in order to determine the vectorquality metric, Block 126 of FIG. 3, for example, the device sets avector quality metric detection window 112, based on the sensed R-wave108 for each of the sensing vectors 102-106, for determining a vectorquality metric associated with the sensing vectors 102-106. According toan embodiment, the device sets a quality metric detection window 112 tostart at a start point 114 located a predetermined distance 116 from theR-wave 108, and having a detection window width 118 so as to allow ananalysis of the signal 100 to be performed in an expected range of thesignal 100 where a T-wave of the QRS signal associated with the sensedR-wave 108 is likely to occur. For example, the device sets the qualitymetric detection window 112 as having a width 118 of approximately 200ms, with a start point 114 of the quality metric detection window 112located between approximately 150-180 milliseconds from the sensedR-wave 108, and the width 118 extending 200 ms from the detection windowstart point 114 to a detection window end point 120, i.e., at a distanceof approximately 350-380 ms from the detected R-wave 108. Once thequality metric detection window 112 is set, the device determines aminimum signal difference 122 between the sensed signal 100 and thesensing threshold 110 within the quality metric detection window 112,i.e., the minimum distance extending between the sensed signal 100 andthe sensing threshold 110. This determined minimum signal difference 122for each of the three sensing vectors 102-106 is then set as the vectorquality metric for the simultaneously sensed R-waves 108 in the sensingvectors, Block 126.

FIG. 5 is a flowchart of a method for determining a morphology metricfor selecting a sensing vector, according to one embodiment. In order todetermine the morphology metric, Block 126 of FIG. 3, the devicedetermines a narrow pulse count, i.e., pulse number, for the R-wave 108.For example, in order to determine the narrow pulse count for eachR-wave 108 associated with the sensing vectors 102-106, the devicedetermines individual pulses associated with the R-wave using knowntechniques, such as described in commonly assigned U.S. patentapplication Ser. Nos. 13/826,097 and 14/255,158, for example,incorporated herein by reference in their entireties. For eachidentified pulse, the device determines whether the width of the pulseis less than a predetermined threshold. In particular, as illustrated inFIG. 5, the device gets a single pulse of the identified pulsesassociated with the R-wave, Block 200, determines a pulse widthassociated with the pulse, Block 202, and determines whether the pulsewidth is less than or equal to a pulse width threshold, Block 204.

In addition to determining whether the pulse width of the individualpulse is less than or equal to the pulse width threshold, Yes in Block204, the device may also determine whether the absolute amplitude of thepulse is greater than an amplitude threshold, Block 206. According to anembodiment, the pulse width threshold may be set as 23 milliseconds, forexample, and the amplitude threshold is set as a fraction, such as oneeighth, for example, of a maximum slope used in the determination ofwhether the slope threshold was met during the aligning of the beat withthe template, described in commonly assigned U.S. patent applicationSer. Nos. 13/826,097 and 14/255,158, incorporated herein by reference intheir entireties.

While the pulse width determination, Block 204, is illustrated asoccurring prior to the amplitude threshold determination, Block 206, itis understood that the determinations of Blocks 204 and 206 may beperformed in any order. Therefore, if either the pulse width of theindividual pulse is not less than or equal to the pulse width threshold,No in Block 204, or the absolute amplitude of the pulse is not greaterthan the amplitude threshold, No in Block 206, the pulse is determinednot to be included in the narrow pulse count. The device continues bydetermining whether the determination of whether the number of pulsessatisfying the narrow pulse count parameters has been made for all ofthe identified pulses for the R-wave beat, Block 210. If thedetermination has not been made for all of the identified pulses, No inBlock 210, the device identifies the next pulse associated with theR-wave, Block 200, and the process of determining a narrow pulse countfor that beat, Blocks 202-208, is repeated for the next pulse.

If both the pulse width of the individual pulse is less than or equal tothe pulse width threshold, Yes in Block 204, and the absolute amplitudeof the pulse is greater than the amplitude threshold, Yes in Block 206,the number of pulses satisfying the width and amplitude thresholds forthe individual R-wave, i.e., the narrow pulse count, is increased byone, Block 208.

Once the determination has been made for all of the identified pulsesassociated with the R-wave, Yes in Block 210, the device sets the narrowpulse count for the R-wave, Block 212, equal to the resulting updatednarrow pulse count, Block 208. In this way, the narrow pulse count forthe R-wave is the total number of pulses of the identified pulses forthe R-wave that satisfy both the width threshold, i.e., the number ofpulses that have a pulse width less than 23 milliseconds, and theamplitude threshold, i.e., the number of pulses that have an absoluteamplitude greater than one eighth of the maximum slope used during thealigning of the beat with the template, for example. The final narrowpulse count from Block 212 is then stored as the morphology metric foreach R-wave.

In this way, after the process is repeated for multiple R-waves sensedalong each of the sensing vectors 102-106 so that both the vectorquality metric and the morphology metric has been determined for thepredetermined threshold number of cardiac cycles for each of the sensingvectors 102-106, such as 15, for example, Block 130, the devicedetermines the selection metrics, Block 132 of FIG. 3, i.e., a vectorselection metric and a morphology selection metric. As illustrated inFIGS. 3 and 4, once the minimum signal difference 122 has beendetermined for all of the predetermined threshold number of cardiaccycles, Yes in Block 130, the device determines a vector selectionmetric for each vector 102-106 based on the 15 minimum signaldifferences 122 determined for that sensing vector. For example,according to an embodiment, the device determines the median of the 15minimum signal differences 122 for each sensing vector and sets thevector selection metric for that sensing vector equal to the determinedmedian of the associated minimum signal differences 122. Once a singlevector selection metric is determined for each of the sensing vectors102-106, the device ranks the vector selection metrics for the sensingvectors 102-106. For example, the device ranks the determined vectorselection metrics from highest to lowest, so that in the example of FIG.4, the diagonal sensing vector 104 would be ranked first since themedian minimum signal difference for that vector was 0.84 millivolts,the horizontal sensing vector 102 would be ranked second, since themedian minimum signal difference for that vector is 0.82 millivolts, andthe vertical sensing vector 106 would be ranked last, since the medianminimum signal difference for that sensing vector is 0.55 millivolts.

Similarly, in order to determine morphology selection metrics in Block132 of FIG. 3, the device may determine an average, a mean or a maximumpulse count of the 15 determined narrow pulse counts for each of thesensing vectors 102-106. Based on the determined average, median ormaximum narrow pulse count for R-waves simultaneously sensed along thesensing vectors 102-106, the device ranks the vectors based on thedetermined morphology selection metrics as being one of a low pulsecount, a medium pulse count and a high pulse count. For example,according to one embodiment, if the average, mean or maximum pulse countassociated with a sensing vector is greater than 5, the final pulsecount for that vector, i.e., morphology selection metric, is determinedto be “high”. If the average, mean or maximum pulse count associatedwith a sensing vector is less than or equal to 5, but greater than orequal to 2, the final pulse count for that vector, i.e., morphologyselection metric, is determined to be “medium”. Otherwise, if theaverage, mean or maximum pulse count associated with a sensing vector isless than or equal to 1, the final pulse count for that vector, I.e.,morphology selection metric, is determined to be “low”.

According to another embodiment, the sensing vectors 102-106 may berelatively ranked based on the morphology selection metric, so that thesensing vector having the greatest pulse count would be identified asbeing “high”, the sensing vector having the second greatest pulse countwould be identified as being “medium”, and the sensing vector having thelowest pulse count would be identified as being “low”,

FIG. 6 is a chart illustrating a method of utilizing determinedselection metrics for selecting a sensing vector, according to anexemplary embodiment. As illustrated in FIG. 6, assuming that the resultof the determination of the vector selection metric, described above, isthat sensing vector 102 is ranked first, sensing vector 104 is rankedsecond and sensing vector 106 is ranked third, and if the sensingvectors 102-106 are relatively ranked based on the morphology selectionmetric, the six possible scenarios are shown, so that the result of themorphology selection metric may be illustrated by any one of sixpossible scenarios. In a first morphology selection scenario, 300,sensing vector 102 is determined to have a low relative narrow pulsecount (i.e., relative to sensing vectors 104 and 106) over the 15cardiac cycles, sensing vector 104 is determined to have a mediumrelative narrow pulse count (i.e., relative to sensing vectors 102 and106), and sensing vector 106 is determined to have a high relativenarrow pulse count (i.e., relative to sensing vectors 102 and 104). In asecond morphology selection scenario, 302, sensing vector 102 isdetermined to have a low relative narrow pulse count over the 15 cardiaccycles, sensing vector 104 is determined to have a high relative narrowpulse count, and sensing vector 106 is determined to have a mediumrelative narrow pulse count.

In a third morphology selection scenario, 304, sensing vector 102 isdetermined to have a medium relative narrow pulse count over the 15cardiac cycles, sensing vector 104 is determined to have a high relativenarrow pulse count, and sensing vector 106 is determined to have a lowrelative narrow pulse count. In a fourth morphology selection scenario,306, sensing vector 102 is determined to have a medium relative narrowpulse count over the 15 cardiac cycles, sensing vector 104 is determinedto have a low relative narrow pulse count, and sensing vector 106 isdetermined to have a high relative narrow pulse count. In a fifthmorphology selection scenario, 308, sensing vector 102 is determined tohave a high relative narrow pulse count over the 15 cardiac cycles,sensing vector 104 is determined to have a low relative narrow pulsecount, and sensing vector 106 is determined to have a medium relativenarrow pulse count. Finally, in a sixth morphology selection scenario310, sensing vector 102 is determined to have a high relative pulsecount over the 15 cardiac cycles, sensing vector 104 is determined tohave a medium relative narrow pulse count, and sensing vector 106 isdetermined to have a low relative narrow pulse count.

FIG. 7 is a flowchart of a method for selecting sensing vectors usingdetermined vector selection metrics and morphology selection metricsaccording to an embodiment. As illustrated in FIGS. 6 and 7, once thevector selection metrics and the morphology selection metrics have beendetermined for the sensing vectors 102-106, the device identifies theresulting first and second ranked vectors, Block 320, which in theexample of FIG. 6 are sensing vectors 102 and 104, and determineswhether the morphology selection metric of one of the correspondingdetermined morphology selection metrics has a “High” pulse count, Block322. In the example of FIG. 6, this occurs, Yes in Block 322, inmorphology selection scenarios 302, 304, 308 and 310, and does notoccur, No in Block 322, in morphology selection scenarios 300 and 306.If neither one of the determined morphology selection metrics associatedwith the first and second ranked vectors is a “High” morphologyselection metric, No in Block 322, the first and second ranked vectorsare selected as the sensing vectors, Block 324.

If the morphology selection metric of either the first ranked vector orthe second ranked vector is a “High” morphology selection metric, Yes inBlock 322, the device sets the other vector as the first ranked vector,Block 326. For example, in morphology selection metric scenarios 308 and310, the second ranked vector, i.e.; sensing vector 104, is set as thefirst ranked vector and sensing vector 102 is set as the updated secondranked vector, and in morphology selection metric scenarios 302 and 304,the first ranked sensing vector, i.e., sensing vector 102 is set(remains) as the first ranked vector.

In order to determine which one of the remaining two sensing vectors ischosen as the second ranked vector, the device then determines whether adifference between the morphology metrics of the updated second andthird vectors is less than a morphology metric difference threshold,Block 328 and whether a difference between the vector metrics of theupdated second and third vectors is greater than a vector metricdifference threshold, Block 330. For example, according to oneembodiment, the device may determine in Block 328 whether the differencebetween the narrow pulse count determined, as described above, for thevector identified as having the “HIGH” morphology selection metric andthe third ranked vector is greater than or equal to three.

By way of illustration, in morphology selection metric scenarios 308 and310, the device determines whether the difference between sensing vector102 and sensing vector 106 is greater than the morphology metricdifference threshold by subtracting the morphology metric, i.e., narrowpulse count, determined above, for the third ranked sensing vector fromthe morphology metric determined for the vector identified as having the“HIGH” morphology selection metric, i.e., sensing vector 102. Similarly,in morphology selection metric scenarios 302 and 304, the devicedetermines whether the difference between sensing vector 104 and sensingvector 106 is greater than the morphology metric difference threshold bysubtracting the morphology metric, i.e., narrow pulse count, determinedabove, for the third ranked sensing vector from the morphology metricdetermined for the vector identified as having the “HIGH” morphologyselection metric, i.e., sensing vector 104.

If the difference between the morphology metrics of the updated secondand third vectors is not greater than a morphology metric differencethreshold, No in Block 328, the first and second ranked vectors areselected as the sensing vectors, Block 324.

Similarly, for example, according to one embodiment, in order todetermine whether a difference between the vector metrics of the updatedsecond and third vectors is less than a vector metric differencethreshold, Block 330, the device may determine whether the differencebetween the minimum signal difference determined, as described above,for the vector identified as having the “HIGH” morphology selectionmetric and the third ranked vector is less than a nominal minimumthreshold, such as 0.10 millivolts, for example.

By way of illustration, in morphology selection metric scenarios 308 and310, the device determines whether the difference between sensing vector102 and sensing vector 106 is greater than the vector metric differencethreshold by subtracting the vector metric, i.e., minimum signaldifference, determined above, for the third ranked sensing vector fromthe vector metric determined for the vector identified as having the“HIGH” morphology selection metric, i.e., sensing vector 102. Similarly,in morphology selection metric scenarios 302 and 304, the devicedetermines whether the difference between sensing vector 104 and sensingvector 106 is less than the vector metric difference threshold bysubtracting the vector metric, i.e., minimum signal difference,determined above, for the third ranked sensing vector from the vectormetric determined for the vector identified as having the “HIGH”morphology selection metric, i.e., sensing vector 104.

If the difference between the vector metrics of the updated second andthird vectors is not less than the vector metric difference threshold,No in Block 330, the first and second ranked vectors are selected as thesensing vectors, Block 324. If both the difference between themorphology metrics of the updated second and third vectors is greaterthan the morphology metric difference threshold, Yes in Block 328, andthe difference between the vector metrics of the updated second andthird vectors is less than the vector metric difference threshold, Yesin Block 330, the updated first and the third vectors are selected asthe sensing vectors, Block 332. For example, assuming both themorphology metric difference threshold and the vector metric differencethreshold are satisfied, Yes in Blocks 328 and 330, in morphologyselection metric scenarios 308 and 310, vectors 104 and 106 are selectedas the sensing vectors, and in morphology selection metric scenarios 302and 304, vectors 102 and 106 are selected as sensing vectors.

In some instances, the morphology selection metric for two or more ofthe sensing vectors 102-106 may have the same ranking. Therefore,according to one embodiment, if two sensing vectors have the samemorphology selection metric, the device may select the first and secondranked vectors from the vector selection metric, i.e., vectors 102 and104 in the example shown in FIG. 7, as the sensing vectors to beutilized. Or according to another embodiment, if the morphologyselection metric for two or more of the sensing vectors 102-106 is“High”, then the device may select the first and second ranked vectorsfrom the vector selection metric, i.e., vectors 102 and 104 in theexample shown in FIG. 7, as the sensing vectors to be utilized. In bothsituations, the sensing vectors 102 and 104 were chosen based only onthe determined minimum signal differences for the sensing vectors102-106, and therefore no updating of the first and second raked sensingvectors would occur.

It is understood that in addition to the three sensing vectors 102-106described above, optionally, a virtual signal (i.e., a mathematicalcombination of two vectors) may also be utilized in addition to, thusutilizing more than three sensing vectors, or in place of the sensingvectors described. For example, the device may generate a virtual vectorsignal as described in U.S. Pat. No. 6,505,067 “System and Method forDeriving Virtual ECG or EGM Signal” to Lee, et al; both patentsincorporated herein by reference in their entireties. In addition,vector selection may be selected by the patient's physician andprogrammed via a telemetry link from a programmer.

In addition, while the use of a minimum signal difference is described,the device may utilize other selection criteria for ranking vectors. Forexample, according one embodiment, the device may determine, for eachvector, a maximum signal amplitude within the detection window for eachR-wave, determine the difference between the maximum amplitude and thesensing threshold for each of the maximum amplitudes, and determine amedian maximum amplitude difference for each sensing vector over 15cardiac cycles. The device would then select the vector(s) having thegreatest median maximum amplitude difference as the sensing vector(s) tobe utilized during subsequent sensing and arrhythmia detection by thedevice.

Thus, a method and apparatus for selecting a sensing vectorconfiguration in a medical device have been presented in the foregoingdescription with reference to specific embodiments. It is appreciatedthat various modifications to the referenced embodiments may be madewithout departing from the scope of the disclosure as set forth in thefollowing claims.

1-21. (canceled) 22: A method comprising: sensing a plurality of cardiacsignals on a plurality of sensing vectors formed from a plurality ofelectrodes, each of the cardiac signals sensed on a respective one ofthe plurality of sensing vectors; determining, for each of the pluralityof sensing vectors, a respective sensing vector metric of the respectivesensed cardiac signal, wherein determining each of the sensing vectormetrics comprises comparing an amplitude of the respective sensedcardiac signal to an amplitude threshold; determining, for each of theplurality of sensing vectors, a respective morphology metric associatedwith a morphology of the respective sensed cardiac signal; determining,for each of the plurality of sensing vectors, a respective vectorselection metric based on at least the determined sensing vector metricand the determined morphology metric; and selecting a sensing vector ofthe plurality of sensing vectors based on at least the determinedselection metrics. 23: The method of claim 22, wherein determining eachof the respective morphology metrics associated with the morphology ofthe respective sensed cardiac signal comprises determining a count ofpulses satisfying at least one narrow pulse criterion in the respectivesensed cardiac signal. 24: The method of claim 22, wherein determiningeach of the respective morphology metrics associated with the morphologyof the respective sensed cardiac signal comprises comparing a pulsewidth of a pulse of the respective sensed cardiac signal to a morphologypulse width threshold. 25: The method of claim 22, wherein determiningeach of the respective morphology metrics associated with the morphologyof the respective sensed cardiac signal comprises comparing theamplitude of a pulse of the respective sensed cardiac signal to amorphology amplitude threshold. 26: The method of claim 22, whereindetermining each of the respective morphology metrics associated withthe morphology of the respective sensed cardiac signal comprises:comparing a pulse width of a pulse of the respective sensed cardiacsignal to a morphology pulse width threshold; and comparing theamplitude of the pulse of the respective sensed cardiac signal to amorphology amplitude threshold. 27: The method of claim 22, whereindetermining each of the sensing vector metrics further comprises:setting a sensing vector window, wherein the sensing vector windowcomprises a start point that is a first predetermined duration after anR-wave of the respective sensed cardiac signal and an end point that isa second predetermined duration after the start point; and comparing theamplitude of the respective sensed cardiac signal within the sensingvector window to the amplitude threshold. 28: The method of claim 27,wherein the start point of the sensing vector window is greater than orequal to 150 milliseconds and less than or equal to 180 millisecondsafter the R-wave of the respective sensed cardiac signal, and whereinthe end point of the sensing vector window is about 200 millisecondsafter the start point. 29: The method of claim 27, wherein the startpoint of the sensing vector window is at a time after an R-wave of therespective sensed cardiac signal and prior to a T-wave of the respectivesensed cardiac signal, and wherein the end point of the sensing vectorwindow is at a time after the T-wave of the respective sensed cardiacsignal. 30: The method of claim 22, wherein comparing the amplitude ofthe respective sensed cardiac signal to the amplitude thresholdcomprises determining a minimum difference between the amplitude of therespective sensed cardiac signal and the amplitude threshold. 31: Asystem comprising: a sense amplifier configured to sense a plurality ofcardiac signals on a plurality of sensing vectors formed from aplurality of electrodes, each of the cardiac signals sensed on arespective one of the plurality of sensing vectors; and processingcircuitry configured to: determine, for each of the plurality of sensingvectors, a respective sensing vector metric of the respective sensedcardiac signal, wherein determining each of the sensing vector metricscomprises comparing an amplitude of the respective sensed cardiac signalto an amplitude threshold; determine, for each of the plurality ofsensing vectors, a respective morphology metric associated with amorphology of the respective sensed cardiac signal; determine, for eachof the plurality of sensing vectors, a respective vector selectionmetric based on at least the determined sensing vector metric and thedetermined morphology metric; and select a sensing vector of theplurality of sensing vectors based on at least the determined selectionmetrics. 32: The system of claim 31, wherein to determine each of therespective morphology metrics associated with the morphology of therespective sensed cardiac signal, the processing circuitry is configuredto determine a count of pulses satisfying at least one narrow pulsecriterion in the respective sensed cardiac signal. 33: The system ofclaim 31, wherein to determine each of the respective morphology metricsassociated with the morphology of the respective sensed cardiac signal,the processing circuitry is configured to compare a pulse width of apulse of the respective sensed cardiac signal to a morphology pulsewidth threshold. 34: The system of claim 31, wherein to determine eachof the respective morphology metrics associated with the morphology ofthe respective sensed cardiac signal, the processing circuitry isconfigured to compare the amplitude of a pulse of the respective sensedcardiac signal to a morphology amplitude threshold. 35: The system ofclaim 31, to wherein determine each of the respective morphology metricsassociated with the morphology of the respective sensed cardiac signal,the processing circuitry is configured to: compare a pulse width of apulse of the respective sensed cardiac signal to a morphology pulsewidth threshold; and compare the amplitude of the pulse of therespective sensed cardiac signal to a morphology amplitude threshold.36: The system of claim 31, wherein to determine each of the sensingvector metrics, the processing circuitry is further configured to: set asensing vector window, wherein the sensing vector window comprises astart point that is a first predetermined duration after an R-wave ofthe respective sensed cardiac signal and an end point that is a secondpredetermined duration after the start point; and compare the amplitudeof the respective sensed cardiac signal within the sensing vector windowto the amplitude threshold. 37: The system of claim 36, wherein thestart point of the sensing vector window is greater than or equal to 150milliseconds and less than or equal to 180 milliseconds after the R-waveof the respective sensed cardiac signal, and wherein the end point ofthe sensing vector window is about 200 milliseconds after the startpoint. 38: The system of claim 36, wherein the start point of thesensing vector window is at a time after an R-wave of the respectivesensed cardiac signal and prior to a T-wave of the respective sensedcardiac signal, and wherein the end point of the sensing vector windowis at a time after the T-wave of the respective sensed cardiac signal.39: The system of claim 31, wherein to compare the amplitude of therespective sensed cardiac signal to the amplitude threshold, theprocessing circuitry is configured to determine a minimum differencebetween the amplitude of the respective sensed cardiac signal and theamplitude threshold. 40: A non-transitory, computer-readable mediumcomprising instructions that, when executed, are configured to causeprocessing circuitry to: sense a plurality of cardiac signals on aplurality of sensing vectors formed from a plurality of electrodes, eachof the cardiac signals sensed on a respective one of the plurality ofsensing vectors; determine, for each of the plurality of sensingvectors, a respective sensing vector metric of the respective sensedcardiac signal, wherein determining each of the sensing vector metricscomprises comparing an amplitude of the respective sensed cardiac signalto an amplitude threshold; determine, for each of the plurality ofsensing vectors, a respective morphology metric associated with amorphology of the respective sensed cardiac signal; determine, for eachof the plurality of sensing vectors, a respective vector selectionmetric based on at least the determined sensing vector metric and thedetermined morphology metric; and select a sensing vector of theplurality of sensing vectors based on at least the determined selectionmetrics. 41: The computer-readable medium of claim 40, wherein todetermine each of the respective morphology metrics associated with themorphology of the respective sensed cardiac signal, the processingcircuitry is configured to determine a count of pulses satisfying atleast one narrow pulse criterion in the respective sensed cardiacsignal.